The present invention relates to an improved streaming current detector and in particular to an apparatus for measuring the average net ionic colloidal surface charge on the surface of an annulus formed of a dielectric material, the charge being a function of the charge influencing species, such as ions, charged molecules or colloidal particles, which are present in a liquid stream in flowing contact with said dielectric surface.
It is well known most waters contain ions and other charged particles species such as colloids and that when certain chemicals are employed to treat the water, contaminates in the water will form aggregates and settle out of the water as a floc. But any floc which forms has a tendency to stick to every surface with which it comes into contact.
A useful quantity for controlling the amount of chemicals needed to treat water is the streaming current, a current generated when a dielectric surface, in the presence of charged species in water, is moved past a pair of electrodes. Unfortunately, if any contaminants are allowed to build up on either the electrodes or a surface disposed proximate to the electrodes, the streaming current cannot be reliably measured.
Many attempts have been made at keeping the electrodes of a streaming current detector clean during operation. One of the latest is disclosed by Bryant and Veal in U.S. Pat. No. 4,769,608. There Bryant and Veal disclosed a self-cleaning probe for a streaming current detector in which the electrodes are mounted within a flowpath member. A test flow stream enters this probe along a transverse passageway. A small portion of the flow is then alternately sucked, by the action of a piston, into capillary-sized channels formed between an inner wall of the flowpath member and the piston and expelled back into the transverse passageway. Leaving the passageway, the flow stream plummets downwardly through housing surrounding the probe before being finally discharged.
The probe and probe housing in the Bryant and Veal device disclosed in U.S. Pat. No. 4,769,608 are designed so that there are no low flow velocity or stagnate areas where solids can accumulate. Floc which does come in contact with the probe is constantly washed off by the rapidly moving flow stream. In spite of this constant washing action, however, the applicants have found that the probe must be cleaned periodically, especially when the probe is used to measure the streaming current of highly contaminated wastewater.
This periodic cleaning is required because the electrodes themselves within the probe tend to foul over time. In the cited prior art combination, cleaning the probe required the operator to turn off the streaming current detector, disassemble it, clean or replace the entire flowpath member, and then reassemble the detector. Much time was wasted, necessitating a costly shutdown of the process which the detector was regulating.